Field of the Invention
The present invention relates generally to techniques for gathering or obtaining information at one or more remote locations and, more particularly, to a specific technique for obtaining from one or more remote locations fluorimetric information, that is, fluorescent light emanating from and characteristic of particular materials at these locations, without utilizing entirely separate information gathering apparatus located at each of the information bearing locations.
Analytical monitoring and control are crucial in a variety of situations arising in medicine, industrial operations, and scientific research. However, in many cases, analysis or information gathering must take place at a distance because the region to be monitored is inaccessible or involves hazardous components, such as high pressures, high temperatures, corrosive materials, high radiation levels, or the like. The need to monitor critical parameters under such conditions arises in direct monitoring of blood parameters, such as pO.sub.2, pCO.sub.2, and pH; in monitoring nuclear reactor vessel parameters, such as temperature, pressure, and in the case of pressurized water reactors or boilers in general, the presence and concentration of certain corrosive ions in the coolant; in monitoring containment of hazardous materials at underground nuclear waste-disposal sites, or chemical dumping sites; in monitoring the quality of ground water; in monitoring reaction conditions at the working zone of a coal-liquification reactor; in monitoring acidity and selected ion concentrations in ore-tailing dumps undergoing microbiological leaching; or in monitoring parameters in like environments which are too hostile or inaccessible for most in situ analytical devices.
In medicine, invasive, or direct, monitoring of blood acid-base parameters and other selected ions is desirable, and in many cases necessary, in the management of critically ill patients or those undergoing complex surgical procedures. In particular, blood pH is regulated within very narrow bounds in normal individuals, varying no more than several hundredth of a pH unit from an average of 7.40. The pH is directly dependent on bicarbonate and dissolved CO.sub.2 concentrations in the blood. As a consequence, several anesthetic agents and diseases affect blood pH, either directly or indirectly. In particular, diabetic acidosis which arises from depletion of serum bicarbonate, and pulmonary disorders and anesthetic agents which affect respiration can cause rapidly increased blood pCO.sub.2, which in turn can produce striking alterations in blood pH. Either of these events are life threatening. Thus, there is an important medical need for directly monitoring blood pH.
Currently, the most widespread methods for direct blood pH measurement, or direct blood electrolyte monitoring, involve the use of ion-selective electrodes. While such electrodes can provide rapid and accurate measurements, there are several disadvantages to their use. The familiar glass pH electrode does not readily lend itself to the construction of invasive devices. Although miniature glass electrodes have been mounted on flexible catheters, small glass electrodes are inherently fragile and therefore present serious risk to the patient. Indeed, most investigators of in vivo blood pH have not employed invasive electrodes, but rather have adopted the somewhat more cumbersome technique in which an arterial-venous shunt is constructed to allow blood flow past a rigidly mounted, mechanically protected glass electrode.
Electrical interference is a major problem with high-resistance microelectrodes such as glass electrodes. Low-resistance miniature electrodes are available, and can give satisfactory measurements in the presence of other electrical equipment, but these require that the amplifying and processing electronics be physically close to the electrodes. Thus, the capability for remote measurements is lost. The most common electrical interference occurs in the 50-60 Hz and radio frequency ranges. While such interference can be reduced by special filtering electronics, both forms of interference can cause DC shifts which are easily overlooked.
Finally, the use of currently available electrodes can present direct hazards to patient safety. Electronically based transducers can pose an electrical hazard, especially when other such transducers are used at the same time, and polyvinyl chloride-based electrodes widely used with ionophores, such as valinomycin, can be dissolved by many gaseous anesthetics.
The electrical interference problem of electrodes is not limited to their uses in medicine. In any environment where high sensitivity is critical, electrical noise generated by extraneous fields will be a problem. Other problems inherent to the use of electrodes include the susceptibility of wire leads and couplings to deterioration under corrosive conditions, or conditions of alternating temperatures.
In the area of industrial process control a host of situations arise where chemical conditions, such as temperature, pressure, pH, redox potential, and ion concentrations, must be continously monitored. Any steam-based power system is susceptible to damage by pH levels which are too high or too low, or by water contaminated by corrosive substances, such as chloride ions, ferrous and ferric ions, copper, and dissolved gases, such as oxygen. Current techniques for monitoring levels of these materials are often indirect, and require that samples of feedwater be removed from the system. For example, water purity is frequently determined indirectly by the sodium tracer technique. In this technique a sample of feedwater is removed from the steam system, and its sodium content is analyzed by a flame photometer. The solids content of the feedwater is then inferred from the amount of sodium detected. Another technique involves measuring the electrical conductivity of the feedwater. Both techniques require that an expensive and complicated sampling system be maintained which is susceptible both to mechanical failure and to human error.
In the area of industrial microbiology, many processes require continous monitoring of chemical conditions. Microbiological leaching of low-grade ores is an area where a need exists for apparatus to remotely monitor parameters, such as oxygen concentration, ferrous and ferric ion concentrations, and pH. Commercial microbial recovery of copper and uranium is currently underway, and processes for recovering zinc, nickel, sulfur and cobalt using similar techniques are currently being considered. A major problem with current leaching processes is that they are carried out on a grand scale and are largely uncontrolled. For example, in a typical dump-leaching operation an entire valley is filled with low-grade ore tailings. As water percolates through the tailings insoluable minerals are made soluable by the action of certain bacteria, such as Thiobacillus ferroxidans and Thiobacillus thiooxidans, and are carried to the mouth of the valley where they are extracted.
The action of the commercially most important leaching bacteria is critically dependent on pH, ferrous and ferric ion concentrations and oxygen concentration. For effective use of such bacteria these parameters must be monitored and controlled. However, because of the lack of inexpensive and rugged sensors, extensive monitoring of large-scale leaching operations is not commercially feasible.
Many of the above-mentioned difficulties with current information-gathering technology can be overcome by using remote, in situ optical probes coupled to a detector by optical waveguides, or fiber optics. Fiber optics are durable, corrosion-resistant, heat-resistant, impervious to electrical or magnetic interference, and are available in very small diameters, which makes them amenable for use with miniature probes.
Peterson, et al., in U.S. Pat. No. 4,200,110, dated April 29, 1980, discloses a remote pH sensing device which employs an optical transducer connected to a detector by two fiber optics. The optical transducer is a membranous-walled chamber or a gel which contains a pH sensitive dye. The dye is illuminated with white light transmitted by one fiber optic, and the light scattered or emitted by the dye molecules is collected by the other fiber optic. Use of more than one fiber optic reduces sensitivity because precise alignment of the illuminating and light-collecting fibers must be maintained, and because illumination of dye molecules is less efficient when separate fibers for illumination and collection are used than if a single fiber is used for both collection and illumination.
Harte in U.S. Pat. No. 3,992,631, dated Nov. 16, 1976 discloses an apparatus for measuring fluorescent emissions from fluorochromes immobilized on a solid surface. The apparatus employs multistrand cables of fiber optics both to transmit an illumination beam and to collect fluorescent emissions. As with Peterson, et al., the sensitivity and reproducibility of Harte's apparatus is reduced by the use of separate fibers for illumination and collection. Also, for long range remote applications the cost of multistrand fiber optic cables can be prohibitive. Because of this its utility as an on-line process monitoring device is severely limited.
The foregoing illustrates some of the limitations of current process-monitoring technology. It is apparent that it would be advantageous to provide alternatives to available methods, particularly in regard to methods for remote, multiposition information gathering in hostile or inaccessible environments.